How sound works in your room - small room acoustics - part 3

So far we have covered how sound works in a room, about room modes and bass trapping, before we look at how we test our room and then how we resolve the issues raised by these tests we are going to look at speaker positioning and the importance of ensuring that you and your speakers are placed in the best possible position within your room. In acoustics, we term the problems created by Speaker positioning as SBIR.

• Read part 1 here
• Read part 2 here

What is SBIR?

Speaker Boundary Interference Response is the term to describe how the proximity of a speaker to a hard boundary (wall/ceiling/floor) will change the response, especially in the low end. In an ideal world all speakers would actually be played in a free field. A free field simply put is a space with no boundaries. In this scenario they would be no absorption, reflection, diffusion, the soundwaves would just decay as intended over the time and space required.

So how does SBIR work and why can it be a problem?

In a nutshell SBIR is primarily a case of distortion or interreference created by reflections from your speakers reflecting back from the wall and when they combine with the direct wave soundwaves it creates interference. As with all soundwaves, the problem occurs when the length of these reflections and the direct soundwaves correspond in wavelengths (and quartiles thereof). If they are half a wavelength out of phase, they cancel each other. Basically, the location of the speakers to the boundary has created another acoustical phenomenon known as COMB FILTERING. A comb filter is when a delayed version of itself (in this case a wavelength) interacts causing either constructive or deconstructive interference. A null is created when the signal is out of phase with each other by half a wavelength. If comb filtering was present when recording drums for example, for the placement of two microphones, the result would lead to a less punchy sounding recording.

So, to put all this together and if we recall what we discussed in my first instalment of on small room acoustics every frequency has a different wavelength. If a delayed version of itself was to reach the listener at the same time we would get comb filtering. These delayed versions are created by reflections mirrored in the boundaries surrounding the loudspeaker. So, the position of the loudspeaker to the boundaries around them is going to change the interference we receive.  The distance the loudspeakers are from these boundaries are going to measurable in both cm’s and in which frequencies are affected. Move them and this interference will move to correspond with another soundwave that corresponds to the new distance.

As I said in my first article, nothing needs to be a problem in acoustics. Anything negative can be turned into a positive. Yes, we know that this interference exists but what we are interested in is how we make this interference work for us rather than against us. We are going to do this by moving the position of the speakers. Potentially nearer the wall, maybe further out but basically, we are going to MANIPULATE that interference until it is either very minimal or in a frequency range that we can easily deal with via normal acoustic treatment and techniques.

Case Study – SBIR In Action 

In a break from the pure science I thought that the best way to really demonstrate SBIR in action was to walk you through a case study. This is an actual GIK Acoustics customer who has been kind enough to allow us to use his experience for this article. As I go through this case study I will be introducing you to 3 different types of acoustical graphs. I will take a little time to talk you through each graph, what it means and what it is representing in this case study. In a later issue we will delve into acoustical testing, how to do it and what it means in more detail.

The Room

This was a dedicated listening room that had already been advised on and treated by GIK Acoustic products. As with all scenarios, the room was treated within the confines of the space, budget and restrictions as agreed with our client. At the front of the room, each corner had floor to ceiling Tri Trap Bass Traps. 2 X GIK 244’s was also on the front wall behind the speakers.

The ceiling was also treated with two clouds and (not in the picture) freestanding traps placed in the first reflection points. In the second picture we see the back of the room. The corners here are inaccessible to treatment but the back wall has 2 X Alpha 2D panels on them to both absorb and diffuse.

The Treated Room Test Results 

On the whole our client was super happy with his room. However, he was experiencing one large problem. A null at 51hz. He ran some REW EQ Wizard tests (a freely available room testing software we highly recommend) and presented us with the following three graphs. I will explain its graph and its meaning as we go.

Above is a Sound Pressure Graph. It measures the loudness of each frequency. What we try to achieve is to get the difference between each frequency as level with each other as possible. As you can imagine, if 60 hz is much louder than 70hz, the 60hz would drown out the 70hz and thus the music would not be reproduced as intended. On the left-hand side, we have the db’s (the loudness). Across the bottom we have the frequency recorded. The problem here is the large null (or lack of loudness) of 51hz.

The above graph only tells us the loudness of each domain. We also want to know what is going on with regards to how long each of those frequencies remain in the room. For our case study we are just going to concentrate on the Spectrogram Graph –

You may have seen in the past a waterfall graph representing decay time. This is similar, it is that waterfall viewed from above. Along the left-hand side, we have how long each frequency rings out in the room in m/s (or the way we look at it, how long it takes to decay). The bottom is again the frequency. As an added dimension to this graph is the colour coding. That is pressure (in db’s like our SPL). Red is the most intense, dark blue the least.

In this scenario we have a ringing at about 37hz for over 800ms and 51hz is nonexistent (this is our comb filtering scenario).

What are we looking for with a spectrogram? Every room is different, but we are looking to reduce the ringing of each frequency to between 140ms and 240ms. As you can see above 80hz we have achieved that in this room. Is it the very low end we are struggling with.

So, what did these graphs tell us? In simple terms the large null then reproduced with no decay time on the spectrogram shows comb filtering in action. There are other graphs and information that we also have provided but all the indications pointed towards it being a problem with SBIR.

Tests Following Moving the Speakers Closer to the Wall. 

This time the SPL graph has both results on it for comparison. As we moved the speakers closer to the wall the results started to change. We had changed the distance to the boundaries therefore the frequency response also changed. It took 10 dbs off that 51hz null.

What about our Spectrogram? We our 37hz problem has been reduced but still exists and that null is still there. Surely, we could do better? Well in this case we could, and this is why acoustics can at times be quite complex and confusing and why we would always recommend getting the advice from a professional where you can. In this case we asked our client to remove the acoustic panels from behind the speakers. There are so many factors that come into play with SBIR, the loudspeaker and how they have been manufactured means that sometimes the only way to tame SBIR is with a 100mm but sometimes having a panel so close to absorption will make it worse. In this case we asked our client to remove the acoustic panels from behind the speakers and place them in the rear corners

So, this time I have left the previous reading on and compared it to our new reading. Look at the difference. The peak at 37 hz has been reduced by 15db’s, the null at 51hz by about 12hz. Overall the room is now looking and sounding pretty good for the treatment it has in it.

Relating this to the Spectrogram –

The difference is even clearer to see. That 37hz decay time has literally been cut in half (if you want to know how we can deal with this further look out for more information in later issues). This room is looking great and this has been achieved by a combination of excellent positioning and acoustic treatment.

Speaker Boundary Interference Response need not ever be a problem, it can certainly be made to work for you rather than against you. What I personally always recommend, and it is indeed one of the most important things you can do when setting up your room is spend the time to position the speakers to get the best results. If you do that first, it makes treating the rest of the room so much easier. Just moving the speakers bit by bit and testing them is the best way to start and from there you can start to build up a picture of how the sound is working in your room, with your boundaries and modes in place.